This PR changes a few small things so I'd happily take some feedback
and chat about whether they are all a net good for the project. I've
done them for myself, but perhaps it improves things for others too.

In summary:

1. The webpack build allows env vars (e.g. from Docker build args) to
set the sheet ID (URL) and name at build time

2. The code itself is refactored a little to allow
dependency-injection of the sheet URL

3. The Dockerfile does the webpack build etc. at build time

4. The Docker build is now multistage which makes it simpler

Rationale
---------

I liked the idea of being able to run the project as it is while also
being able to run a build, even from my own Dockerfile elsewhere, that
spits out an HTML file that is pre-baked with the radar data. This
would allow deploying it as part of a wider static site, e.g. on
Github pages.

It seems a reasonable code architecture to hoist out mentions of
`window` to the top-level file and inject that info in such that all
JS files from `factory.js` are abstracted away from the fact that info
came from a query string.

Docker Build Changes
--------------------

The Docker container running the bulk of the build as a `CMD` on start
seemed odd when -- as far as I can see -- the build output is entirely
a function of source files alone.

Maybe there's more flexibility intended around being able to swap out
env vars without having to rebuild the container?

I technically do not need the changes for the sheet URL injection, but
I'd have to add something to pass the build args into the environment
at container runtime.

Small Tweaks
------------

Some stuff I noticed along the way:

1. There's two webpack configs but they're _very_ close so I hoisted
up the `entry` part to the common file. There may be a reason it is
the way it is.

2. I kept finding it odd to have lots of references to "sheet" and
"Google Sheet Input" throughout when it's clearly been extended to
allow generic JSON and CSV outside of Google sheets. I suspect this is
historic but I changed the name of the top export from factory in the
hope this is a positive change in the right direction. I can revert if
needed though.

3. For JSON and CSV input, the `h1` title just uses the file name.
Since we can now inject I've allowed a SHEET_NAME to be set at build
time to override that behaviour.

Tests
-----

I've checked no tests fail, but not added any since the JS changes are
_meant_ to be a refactor but perhaps this change warrants an e2e test
to verify the new functionality made available.